In a message dated 3/13/01 9:14:09 AM Pacific Standard Time,
n4zr@contesting.com writes:
> I've asked this related question before, and maybe it falls under the
> heading of "no matter nohow," but I think it's generally understood that
> the pretensioning of guy wires is intended to take them from
> "take-out-the-droop" mode to "stretch" mode.
"Pre-tensioning" isn't a concept covered by the TIA-222-F, "Structural
Standards for Steel Antenna Towers and Antenna Supporting Structures" which
is the tower design bible. They do define "initial guy tension" as "the
specified guy tension in pounds [newtons] under no wind load conditions, at
the guy anchor at the specified temperature".
You may be thinking of "pre-stressing" which is "the removal of inherent
constructional looseness of the guy under a sustained load". This is
unnecessary for small diameter guys like typically used for ham
installations.
When you're constructing a guyed tower, there is an initial tension that
is a moderate amount used when the guys are first attached to the anchors so
that you can equalize them and then plumb the tower. I use 2-3% or so (not
critical) of the ultimate breaking strength (UBS).
What you're describing is just "tensioning".
> Since Phillystran is
> significantly lighter than EHS, it should approach a straight line under
> less tension than EHS does. For that reason, couldn't it be used with less
> pre-tensioning than EHS, giving slightly greater ultimate load capability?
As the tension on a wire rope-type material is increased, the closer its
characteristics get to a solid rod. Lighter doesn't make much difference and
it has no bearing on the UBS.
>
> And yes, I know that it is spoecced for the same pre-tensioning as EHS, but
> I'm just wondering whether that's correct, or just seat-of-the-pants by
> some spec writer...
Some 'spec writer' being the Telecommunications Industry Association
which is analogous to ANSI, SAE, etc.
While the UBS for different sizes of EHS are widely known, the guy
tension per temperature is not. Here's a table for guy tension that appeared
in my column "Up The Tower" in CQ Contest magazine:
Table B: Temperature change chart for 3/16" and 1/4 " EHS guy tension
Temp F 3/16" EHS 1/4 " EHS
120 degrees 300 pounds 501 pounds
90 degrees 350 pounds 583 pounds
60 degrees 400 pounds 670 pounds
30 degrees 450 pounds 747 pounds
0 degrees 500 pounds 830 pounds
In all things, follow the LXC Prime Directive (DO what the manufacturer
says) and you'll be just fine.
Cheers, Steve K7LXC
Tower Tech
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